The invention relates to a method of forming a field-effect transistor in a semiconductor substrate. For connecting the field-effect transistor to a first conductivity region of the semiconductor substrate, a connection region to a source-drain region of the field-effect transistor is formed. For forming a potential source-drain channel path of the field-effect transistor device, an overlapping region of the source-drain region and/or of the connection region is provided with a gate insulating region.
In the case of many semiconductor circuit configurations, appropriate field-effect transistors have to be provided in a semiconductor substrate to form and connect up the underlying semiconductor circuit configuration. In particular, field-effect transistors of this type serve for connecting to a conductivity region, for example a bit line connection, an electrode connection or the like, of the semiconductor substrate, with a corresponding connection region, for example a buried-strap region, being provided for the connection of the source-drain region of the transistor to a top electrode of a storage capacitor. Furthermore, a spatial overlapping region of the source-drain region of the field-effect transistor is provided with a gate insulating region, to allow a source-drain channel path of the field-effect transistor device to be formed during operation.
The corresponding connection regions and/or the overlapping regions with the gate insulating region are usually formed as what are known as diffusion regions or diffusion contacts, with appropriate dopant material and consequently charge carriers being distributed in a thermally induced manner in the semiconductor substrate with different concentrations from a material region provided as a dopant depot.
It is problematical in this case that the underlying diffusion processes for forming the charge carrier distributions in a given material region proceed substantially isotropically. If a vertical direction of propagation is preferred, for example for contacting with vertical transistors, this results in that a lateral diffusion of the charge carriers nevertheless also takes place, with the result that under some circumstances countermeasures have to be taken to avoid undesired instances of contacting. The countermeasures have until now been realized by a greater spacing between conductivity regions or components that are to be insulated from one other. On account of this necessity to maintain minimum spacings, there are limits to the objective of highest possible component integration in an extremely small space.
It is accordingly an object of the invention to provide a method of forming a vertical field-effect transistor that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which undesired instances of contacting of neighboring components of highly integrated circuits can be prevented in a simple way.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of forming a field-effect transistor device. The method includes providing a semiconductor substrate, forming a connection region in the semiconductor substrate, forming a conductivity region in the semiconductor substrate, and forming a source drain region. The connection region extends to the source-drain region for connecting the field-effect transistor device to the conductivity region. A potential source-drain channel path of the field-effect transistor device is formed by providing a gate insulating region to an overlapping region of the source-drain region and/or the connection region. The overlapping region is formed directly as a material region.
The method according to the invention of forming a field-effect transistor in a semiconductor substrate is characterized in that the respective overlapping region, in particular the source-drain region, of the field-effect transistor device is formed directly as a material region, in particular with lateral outdiffusion processes being avoided to the greatest extent.
Consequently, a fundamental idea of the present invention is to form and/or deposit the overlapping region of the connection region and/or of the source-drain region directly as a material. This takes place in contrast to conventional procedures, in which an overlapping region is initially formed in an electrically insulating manner, with the electrical contacting subsequently taking place indirectly, that is by outdiffusion of appropriate dopant material and consequently by subsequent introduction and distribution of corresponding charge carriers into the semiconductor region, which in itself is electrically insulating. With the direct material-based and electrically conducting contacting of the gate insulating region or gate oxide region with the connection region, the buried-strap region, or the source-drain region of the field-effect transistor, there is no longer the necessity for the subsequent distribution of charge carriers by outdiffusion, because an electrically conductive material region or a corresponding region for the electrical contacting of the gate insulating region with the remaining regions is provided directly.
In the case of a particularly preferred embodiment of the method according to the invention, the connection region and the overlapping region are formed substantially from the same material.
It is further preferred that the connection region and the overlapping region are formed substantially as one part or as one piece, whereby the process sequences can be made particularly simple and difficulties with regard to material compatibility are then avoided.
According to a further preferred embodiment of the method according to the invention, it is provided that the connection region and/or the overlapping region are grown on epitaxially and/or are formed as a single crystal, in particular as a selective epitaxial single-crystal doped silicon or the like.
According to a further embodiment of the method according to the invention, it is provided that, to avoid or suppress outdiffusion processes from the connection region and/or the overlapping region and/or to avoid or suppress undesired electrical contacts of the connection region and/or of the overlapping region with further regions, the connection region and/or the overlapping region are at least partially enclosed in an insulating region.
It is particularly advantageous if the connection region and/or the overlapping region are formed as a buried-strap structure, as a buried-strap region and/or as part thereof, in particular in a substantially buried form.
According to a further embodiment of the method according to the invention, the respective field-effect transistor device is formed as a vertical trench-structure transistor device, in particular as a deep-trench transistor or the like and/or in particular for connecting to and/or for a DRAM memory cell or the like.
For this purpose, a recess or a trench is respectively formed in the semiconductor substrate.
In an upper trench portion with upper edge regions or upper wall regions, the vertical trench-structure transistor device is formed with gate regions that are substantially electrically insulated with respect to the upper edge regions or upper wall regions and with respect to a middle trench portion to be provided.
It is further provided that, spatially adjoining the insulated gate region, substantially directly, in particular spatially beneath it, the source-drain region and the connection region are provided and formed in the region of the middle trench portion.
Furthermore, it is provided that the first conductivity region to be contacted is provided and formed substantially in a lower trench portion for forming a substantial electrical conducting contact at least with the connection region.
According to a particularly preferred embodiment of the method according to the invention, it is provided that a mask region is deposited on a surface region, which may be formed in a substantially planar manner, of the semiconductor substrate, if appropriate formed in a multi-layered manner, and is structured with recesses in accordance with a desired trench structure to be formed.
Furthermore, it is provided that a single-crystal semiconductor material is used as the semiconductor substrate, in particular a single-crystal silicon, if appropriate p-doped, and/or the like.
It is particularly advantageous for the method according to the invention that, if appropriate, the respective recess and/or the respective trench are structured in a number of process steps temporally separated and/or interrupted from one another, so that in the meantime other structuring processes can be included in an advantageous way.
It is provided in this case that the upper trench portion with upper edge regions or wall regions is etched into the semiconductor substrate, in particular with a depth in the range of 1 xcexcm, in particular for the vertical trench-structure transistor device, with a first temporary bottom region being formed in the recess or in the trench.
The upper trench portion is widened, in particular laterally, preferably by a substantially isotropic etching-back step and/or in particular by a silicon pull-back process, preferably in the range of a layer thickness of about 5 nm.
A first protective layer or insulating layer is deposited, in particular in a conformal manner and/or in particular with a layer thickness in the range of about 5 nm, preferably as an Si3N4 liner or the like.
Then, furthermore, by etching, in particular anisotropic etching, with penetration of the first protective layer at the first temporary bottom region of the recess or trench, the second, middle trench portion with middle wall regions or edge regions is formed, in particular for the connection region, for the source-drain region and/or for the overlapping region or for parts thereof, preferably with a depth in the range of about 200-300 nm, with a second temporary bottom region being formed in the recess or in the trench.
The middle trench portion is widened, in particular laterally, to be precise preferably by an isotropic etching-back step and/or in particular by a silicon pull-back process, preferably in the range of a layer thickness of about 20 to 40 nm, thereby creating recesses in the region of the middle side walls, edge regions or wall regions of the middle trench portion, in particular for the connection region, source-drain region and/or overlapping region to be provided, or parts thereof.
For forming the insulating region for the connection region, the source-drain region and/or the overlapping region and/or for parts thereof, a second protective layer or insulating layer is formed, in particular in a conformal manner, with a layer thickness in the range of about 10 nm and/or in particular using silicon dioxide SiO2, oxidized silicon nitride Si3N4 and/or the like.
Furthermore, for forming the connection region, the source-drain region and/or the overlapping region and/or parts thereof, it is provided that a single-crystal seed region is formed, in particular for forming the first-mentioned regions as selective semiconductor material regions grown on epitaxially as a single crystal.
The single-crystal seed region is formed by etching, in particular anisotropic etching, with penetration of the second protective layer or insulating layer at the second temporary bottom region of the second or middle trench portion, exposing underlying single-crystal semiconductor substrate at the second temporary bottom region, with the further option that the second temporary bottom region is easily removed by etching in the sense of an overetching process. For forming the buried strap in the sense of a connection region, source-drain region and/or overlapping region or for parts thereof, it is provided that single-crystal semiconductor substrate is grown in a selectively epitaxial manner on the single-crystal seed region in such a way that the middle trench portion is completely filled. In particular, the middle trench portion is filled to just above the level of the first temporary bottom region of the recess or the trench, with a varying dopant concentration being introduced if appropriate, in particular in the form of a concentration gradient, and/or the lateral recesses in the region of the second, middle trench portion for the buried-strap region as a connection region, source-drain region and/or overlapping region and/or parts thereof in the region of the middle wall regions or edge regions being completely filled in particular.
The middle trench portion, which was closed by the epitaxial growing and filling, is opened again, to be precise to just below the level of the second temporary bottom region, in particular by anisotropic etching, with material of the second protective layer or insulating layer in the region of the upper edge regions or wall regions being removed in particular.
A third protective layer or insulating layer is formed, so that the second temporary bottom region and, in the region of the middle wall regions or edge regions of the middle trench portion of the recess or of the trench, the selectively epitaxially grown-on single-crystal material in the recesses for the buried-strap region, that is for the connection region, the source-drain region and/or the overlapping region or for parts thereof, is covered.
Preferably, the forming of the third protective layer or insulating layer takes place by thermal oxidation. Selective electrochemical depositing and/or transforming, in particular by oxidation of the silicon material, is also conceivable.
Following that, the third or lower trench portion of the recess or of the trench is then formed by etching, in particular anisotropic etching. Following that, a trench capacitor, for example, can then be formed in the region of the lower trench portion. This takes place, for example, by forming a buried-plate structure as the bottom electrode, subsequent filling with a node dielectric and subsequent introduction of a top electrode. Then, the first conductivity region is created, for example as part of the top electrode or as a connection for it, in this third or lower trench portion, preferably to a level below the first temporary bottom region, preferably by filling the trench with polysilicon or the like and appropriate anisotropic etching back to the desired level. A fourth protective layer or insulating layer is provided if appropriate between the semiconductor substrate and the material of the first conductivity region.
The material of the third protective region or insulating region is selectively removed, in particular in the region covering the selectively epitaxially grown-on single-crystal material in the recesses for the buried-strap region, from the regions, in particular by selective, wet-chemical etching.
This produces a corresponding gap or spacing between the material of the first conductivity region and the epitaxially selectively grown-on single-crystal material for the buried-strap region. The first conductivity region is electrically contacted with the connection region or the buried-strap region, in particular by filling the corresponding gaps with polysilicon and/or by selective epitaxial re-growing and closing of the gaps between the first conductivity region and the material of the connection region or the buried-strap region, with at least part of the middle wall regions or edge regions of the middle or second trench portion of the trench or of the recess, that is in particular of the connection region, remaining free and uncovered.
The surface region of the first conductivity region is then covered with a fifth protective layer or insulating layer, in particular of silicon dioxide SiO2, with in turn part of the middle wall regions or edge regions of the middle trench portion of the trench or of the recess, that is the connection region, remaining free and uncovered. The fifth protective layer or insulating layer on the surface region of the first conductivity region serves classically as what is known as a trench top oxide (TTO).
It is further provided that, by etching, in particular isotropic etching, at least in the region of the transition between the middle trench portion and the upper trench portion and in particular in the recesses for the buried-strap region, that is in the recesses for the connection region, the source-drain region, the overlapping region and/or for parts thereof, respectively existing insulating layers or protective layers between the buried-strap region and the semiconductor substrate are removed, with a substantially laterally extending recess being created in particular in the upper region of the buried-strap region or connection region.
A material-based electrical contact between the remaining buried-strap region or connection region and the semiconductor substrate provided above it must be formed by filling the recess in the upper region of the connection region with an electrically conductive material.
In this case, the filling may be performed by selective epitaxial growing or re-growing or as part of a hydrogen reflow process.
After producing the filling, the filled region substantially forms the overlapping region to be provided with respect to the gate region to be provided and to be insulated.
For this purpose, to form the gate region of the trench-structure transistor device, a sixth insulating layer is deposited as a gate insulating region, in particular in the form of an oxide or the like, and/or in particular at the regions remaining free of the upper and/or middle edge regions or wall regions of the middle and upper trench portions of the trench. In this case, the volume of the trench or of the recess remaining free is filled with a second conductivity region as a gate contact.
These and further aspects of the present invention emerge from the comments now made.
The invention describes an integration sequence that realizes a DRAM cell which is formed completely in a deep trench, with self-adjusting Epi buried strap and without buried-strap outdiffusion and for the connection to the source-drain region of the VFET, and with the thermally induced BS outdiffusion being suppressed laterally by a dielectric shield and (optional) doping gradient in the Epi-layer.
The invention is based on the xe2x80x9cCFExe2x80x9d concept, of the buried collar region and, integrated in this concept or in comparable DT concepts for vertical DRAM cells, an epitaxy-based buried strap (BS) or connection region.
Previous standard concepts (CCT, BPC) provide a sequential production of the DT, buried plate, node dielectric, poly fill, collar and buried strap in various sequences. In the case of the CCT concept, the sequence is used as just described. In the case of the BPC concept, the collar formation takes place between the DT etch and buried plate forming by using, inter alia, a dummy filling of polysilicon.
A distinctly different, innovative and very well scalable concept with a buried collar (CFE) integrates the realization of the collar into the DT etch, which is interrupted twice, in order to define the upper edge and the lower edge of the collar.
The central point of the formation of a buried strap has until now been achieved exclusively by outdiffusion from a highly doped poly filling through a SiN interface, supported by an implantation step. The (buried) strap is the critical element of a DRAM cell. Such a buried strap, as could be used advantageously for vertical trench concepts (VT cells) in particular, is described below.
The invention solves the given problem by integrating an epitaxy-based buried strap (BS) into the DT side wall. The buried strap sits on a diffusion barrier. The upper and lower delimitation of the buried strap is defined by interruptions in the DT etch. In this case, the connection region (buried strap) and drain of the selection transistor merge into one another and can accordingly no longer be geometrically separated from one another.
The following are obtained as additional simplifications:
a) suppression of the lateral outdiffusion from the BS region when there is low formation of the diffusion barrier (barrier heightxe2x88x92100-200 nm);
b) complete suppression-of the lateral outdiffusion and realization of flat VFET junctions when there is high formation of the diffusion barrier (height  greater than  greater than 200 nm); in this case, the risk of punch-throughs is also reduced, since the VFET is partly or entirely surrounded by a dielectric envelope (preferably SiO2); SOI-similar behavior of the VFET;
c) reduction of the overall BS resistance by forming of a large overlapping area between the DT fill and the highly doped BS region; as a result: uncritical overlap between the upper edge of the DT fill and the BS region;
d) simplification of the collar; and
e) drain region of the transistor defined by Epi region of the buried strap; freedom of choice in the dopant concentration or the dopant gradient in the Epi layer makes a low BS greater than drain junction resistance possible.
An inventive step lies in the burying of the buried strap region into the DT side wall by a pull-back method and subsequent selective-epitaxial filling of the BS region with highly doped n-polysilicon. The single-crystal epitaxial growth is possible in concepts in which the DT etch is interrupted, so that there is still single-crystal silicon on the bottom. In this case, the upper and lower edges of the BS region are defined by interruption of the DT etching.
After the etching of a short DT, a protective layer is deposited, protecting the existing DT side wall from the later pull-back step. The layer may be, for example, a nitride. An xe2x80x9cONOxe2x80x9d or an xe2x80x9cNOxe2x80x9d would satisfy this purpose. To protect the protective layer from the subsequent RIE steps, it may be advantageous to widen the DT slightly before the depositing of the layer.
After the silicon pull-back, which exposes the region in which the buried strap is ultimately to be located, an insulator layer is deposited, which in turn may be of, for example, SiO2, Si3N4, xe2x80x9cNOxe2x80x9d, xe2x80x9cONOxe2x80x9d, SiON. The layer is intended to prevent outdiffusion of dopant from the BS into the surrounding p-well. The shielding function can be finely set by the height of this layer (corresponds to the height of the epitaxial BS region). This layer is subsequently opened at the bottom by an RIE step.
By a selective epitaxial step, beginning from the bottom, the recess for the entire buried-strap region is filled. This may involve creating a dopant gradient, to finely set the BS, drain and channel properties. As a particularly extreme case, it should be mentioned here that the shield can also rise up completely over the entire VFET, so that possible junction problems caused by the Epi-Si less than xe2x88x92 greater than Si junction can be minimized.
Then the DT is completed, the (buried) collar is constructed, the buried plate is diffused in, the dielectric is deposited and the DT is filled, for example with highly doped polysilicon, for example as a top electrode.
The connection between the trench fill and the Epi BS is established by a further selective epitaxial step (undoped) or by a slight H2 reflow.
The filling of the trench top oxide (TTO) follows, followed by the wet-chemical opening of the shield at the upper edge.
Then the connection of the BS/drain region to the rest of the VFET takes place by a further, undoped selective Epi step or a slight H2 reflow step. Subsequently, the cell can be realized in accordance with a desired VFET concept.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of forming a vertical field-effect transistor device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.